This study is designed to test the general hypothesis that muscle sympathetic nerve activity (MSNA) and vasoconstrictor responses to sympathoexcitatory maneuvers differs during the follicular versus the luteal phase of the menstrual cycle in healthy young women. Baroreceptor sensitivity is determined by the relationship between the change in diastolic blood pressure (DBP) and MSNA during sequential infusions of nitroprusside (a vasodilator) and phenylephrine (an alpha-agonist vasoconstrictor). Vascular transduction is assessed by the MSNA-calf vascular resistance (CVR) relationship during ischemic handgrip exercise and post-exercise ischemia. Preliminary data suggests that baroreceptor sensitivity is greater during the luteal phase of the menstrual cycle in normally cycling women. This suggests that a decline in DBP during the luteal phase results in a greater increase in sympathetic outflow compared to the follicular phase. Moreover, estrogen concentration appears to be the primary factor that augments baroreceptor sensitivity, and it appears that progesterone antagonizes this effect. Furthermore, vascular transduction may also be augmented during the luteal phase. This would suggest that the vasoconstrictor response to an increase in sympathetic outflow is greater during the luteal phase. It is our goal to continue these studies in more subjects, and to further investigate the impact of oral contraceptive use on these relationships. The second phase of this experiment, to determine the role of the menstrual cycle on orthostatic tolerance, is planned to begin mid-summer 1999.